1. Field
The present invention generally relates to ion conducting devices, and more particularly to current based ion conducting devices arranged into a stack of electrically interconnected solid electrolyte plates.
2. State of the Art
Ion conducting devices based on solid electrolytes are typically constructed according to two basic designs Passive ion conducting devices detect the electromotive force potential across the solid electrolyte due to the Nernst voltage generated when different ion concentrations exist on opposite sides of a membrane. Passive devices are used in a variety of applications including automotive sensors and process gas analyzers. Current based ion conducting devices depend on the transport of ions through the electrolyte due to either a pressure differential across the electrolyte, or from an applied electromotive force. Current based devices are more complex in construction and operation, but have a wider range of applications. Well known applications for current based devices include fuel cells, inert gas purifiers, oxygen concentrators, and steam electrolyzers.
An electrode material is attached to the electrolyte surfaces, the composition of which depends on the intended application and operating conditions. Reactant gases contacting the electrode undergo a reaction whereby an ion species is generated which migrates through the electrolyte. For example, in an oxygen concentrator the ion species is O.sup.-2 , and is formed according to the following reaction: EQU O.sub.2 (from air) +4e .fwdarw.2O.sup.-2
The O.sup.-2 migrates through the electrolyte due to an applied electromotive force. Pure oxygen may be collected on the opposite side of the electrolyte according to the following reaction
: EQU 2O.sup.-2 .fwdarw.2O.sub.2 +4e .sup.-
In a fuel cell, an electrical potential is produced between a cathode and anode in contact with different concentrations of ions. When the electrodes are connected in an electrical circuit, the electrons released at the anode furnish an electrical current.
Several U.S. patents teach solid oxide electrolytes arranged in various structural configurations. U.S. Pat. No. 4,664,987 (Isenberg) teaches a fuel cell arrangement utilizing a tubular configuration for the solid oxide electrolyte. U.S. Pat. No. 4,950,562 (Yoshida et al.) teaches a flat plate solid oxide electrolyte configuration. Several U.S. Patents teach fuel cell assemblies comprising a stack of solid oxide electrolyte elements. Notable among these patents are U.S. Pat. Nos. 5,069,987; 5,049,459; 5,034,288; and 4,950,562. In each of these arrangements, successive electrolyte elements are separated from the elements above and below it by an inert interconnector plate, or separator. The interconnector plates prevent reactants and products from mixing, yet permit the reactants to come in contact with the electrode surfaces. U.S. Pat. No. 4,877,506 (Fee et al.) teaches an oxygen pump employing a stack of solid oxide electrolyte elements, and utilizing inert interconnector plates between successive elements.
The interconnectors in these current based devices are part of an electrical circuit, and must be either constructed from an electrically conductive material, or include an electrical pathway through the interconnector. Electrically conductive materials suitable for incorporation into a flat plate ion conducting device are less expensive than the electrolyte material. The interconnectors may be constructed from the expensive electrolyte material, in which case the electrical pathway is either incorporated into the interconnector, or attached to the exterior. If the solid electrolyte is used, extremely expensive material is used for inert plates comprising approximately 50% of the stack. If an electrically conductive material is used, dissimilar materials are incorporated into the stack.
Several problems inherent in these stacked arrangements make them unreliable, inefficient, and expensive. A typical operating temperature range for solid oxide ion conducting devices is 800-1000.degree. C. Substantial thermal expansion of materials occurs when materials are heated to these high temperatures. The different rate and extent of thermal expansion between dissimilar materials can cause cracks and distortions in the stack. Leaks may develop between the electrolyte and interconnectors, allowing reactant gases to escape before contacting the electrode. Product gases may also escape through cracks in seams or materials. These factors severely reduce the efficiency and reliability of the device. If delamination between stack components is severe enough, total failure of the device may result. Another disadvantage of using dissimilar materials is the difficulty in maintaining an intact electrical interconnect system. Problems with contact resistance frequently result when the interconnects are bonded to different materials. The different coefficients of thermal expansion tend to crack and destroy the conductive material. These problems are obviously undesirable in space applications, where the device must remain reliable for an extended period of time, while exposed to extreme thermal and vibration stresses.
Interconnectors also significantly reduce the efficiency of a stack of electrolyte elements. Because the plates are inert, approximately 50% of the components of the stack are inactive. The interconnectors also must seal against the electrolyte plates. Wherever the interconnector contacts the electrode, gas transport through the electrode to the electrolyte surface tends to be limited. The reactive electrolyte area available for ion conduction is thus potentially reduced through the use of inert interconnectors.
Interconnectors made from electrolyte material contribute significantly to the cost of an ion conducting device. The material itself is very expensive, and approximately one interconnector is required for every active electrolyte plate. Complex manufacturing methods are also required to produce an interconnector with a typical dual ribbed structure. These factors make manufacture of ion conducting devices with interconnectors more difficult and complicated, which increases both cost and manufacturing time.
There is a need for an improved solid oxide electrolyte ion conducting device which overcomes the aforementioned problems.